Editing Greywacke (section)

== Formation ==
The origin of greywacke was unknown until [[turbidity current]]s and [[turbidite]]s were understood, since, according to the normal laws of [[sedimentation]], [[gravel]], [[sand]] and [[mud]] should not be laid down together. [[Geologist]]s now attribute its formation to submarine [[avalanche]]s or strong turbidity currents. These actions churn [[sediment]] and cause mixed-sediment slurries, in which the resulting deposits may exhibit a variety of sedimentary features. Supporting the turbidity origin theory is the fact that deposits of greywacke are found on the edges of the [[Continental shelf|continental shelves]], at the bottoms of [[oceanic trench]]es, and at the bases of mountain formational areas. They also occur in association with black [[shale]]s of deep-sea origin.

As a rule, greywackes do not contain [[fossil]]s, but organic remains may be common in the finer beds associated with them. Their component particles are usually not very rounded or polished, and the rocks have often been considerably [[indurated]] by [[Recrystallization (geology)|recrystallization]], such as the introduction of interstitial [[silica]]. In some districts, the greywackes are cleaved, but they show phenomena of this kind much less perfectly than the slates.{{sfn|Flett|1911}}

Although the group is so diverse that it is difficult to characterize mineralogically, it has a well-established place in [[Petrology|petrographical]] classifications because these peculiar composite [[arenite|arenaceous]] deposits are very frequent among [[Silurian]] and [[Cambrian]] rocks, and are less common in [[Mesozoic]] or [[Cenozoic]] strata.{{Citation needed|date=August 2007}} Their essential features are their gritty character and their complex composition. By increasing [[metamorphism]], greywackes frequently pass into mica-[[schist]]s, chloritic schists and sedimentary [[gneiss]]es.{{sfn|Flett|1911}}

=== Varieties ===
The term "greywacke" can be confusing, since it can refer to either the immature (rock fragment) aspect of the rock or its fine-grained (clay) component.

Greywackes are mostly grey, brown, yellow, or black, dull-colored sandy rocks that may occur in thick or thin beds along with [[shale]]s and [[limestone]]s. Some varieties include ''feldspathic greywacke'', rich in [[feldspar]], and ''lithic greywacke'', rich in other tiny rock fragments. They can contain a very great variety of [[mineral]]s, the principal ones being quartz, [[orthoclase]] and [[plagioclase]] feldspars, [[calcite]], [[iron oxide]]s and graphitic, carbonaceous matters, together with (in the coarser kinds) fragments of such rocks as [[felsite]], [[chert]], [[slate]], [[gneiss]], various [[schist]]s, and [[quartzite]]. Among other minerals found in them are [[biotite]], [[Chlorite group|chlorite]], [[tourmaline]], [[epidote]], [[apatite]], [[garnet]], [[hornblende]], [[augite]], [[sphene]] and [[pyrite]]s. The cementing material may be [[siliceous]] or [[argillaceous minerals|argillaceous]] and is sometimes [[calcareous]].{{sfn|Flett|1911}}